Glass substrates are widely used for displays of liquid crystal displays, OLED displays, etc., substrates of hard disks, filters, sensors, etc., cover glasses of solid state image pickup devices such as CMOS, and the like. In particular, active matrix displays in which pixels are driven by active elements, typified by thin-film transistors (hereinafter referred to as TFT(s)) form the mainstream of liquid crystal displays and OLED displays, and are widely used in the displays for displaying color images and movie images of liquid crystal televisions, notebook computers, liquid crystal monitors, cellular phones, and digital cameras. In such active matrix displays, micron-order high definition electronic circuits such as TFT elements and signal lines are formed on the surface of a glass substrate using a thin film.
The glass substrate for use in the above applications require various properties described below (see Patent Document 1 if needed):
(1) the glass substrate is required to be substantially free of an alkali metal oxide because when the glass has an alkali metal oxide, alkali ions are dispersed in a film-formed semiconductor material during a heat treatment, which leads to degradation of film properties;(2) the glass substrate is required to have such a chemical resistance that the glass substrate is not deteriorated by chemicals (various acids, alkaline solutions, etc.) used in a photoetching process;(3) the glass substrate is required to have a high strain point in such a manner that the substrate is not thermally contracted by heat treatment processes such as film formation, annealing, etc;(4) the glass substrate is required to have a small density so as to achieve lightweight displays; and(5) the glass substrate is required to be matched to the thermal expansion coefficient of peripheral members.    Patent Document 1: Japanese Patent Application Laid-open No. 2000-302475